1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a program which are suitable for use in recursive processing for noise reduction.
2. Description of the Related Art
X-ray fluoroscopic images are taken with very low X-ray doses to minimize radiation exposure adverse-effects. Accordingly, many quantum noises are superposed on the taken X-ray fluoroscopic images. Hitherto, noise reduction of the taken images has been achieved by applying recursive filtering to perform smoothing of pixels in a time direction. The recursive filtering (also referred to as inter-frame noise reduction processing) is very effective in reducing noises in a plurality of still images. However, when the recursive filtering is applied to moving images, image lags can be generated. Therefore, the recursive filter is not always effective in reducing noise in moving images.
Thus, Japanese Patent Application Laid-Open No. 1-273487 has discussed a digital X-ray apparatus capable of displaying a clear image with a smaller image lag even in a case where recursive filtering is performed on a moving image or a continuous image. This digital X-ray apparatus has a separation circuit unit, a filter circuit unit, and an addition circuit unit to achieve noise reduction processing with less image lag. This separation circuit unit separates each frame of an X-ray fluoroscopic image into a low-frequency component image and a high-frequency component image. The filter circuit unit performs recursive filtering on the high-frequency component image. The addition circuit unit adds the separated low-frequency component image to the high-frequency component image on which the recursive filtering has been performed.
Japanese Patent No. 3158371 discusses a noise reducer for reducing noise, which extracts a noise signal from a difference signal after generation of the difference signal representing a difference between a moving image reproduced by an analog video tape recorder and an image obtained by a recursive filter, and which subtracts the extracted noise signal from the reproduced moving image. This noise reducer detects motion of an image with a simple method, and changes a reduced amount of noise between the moving image and a still image. Consequently, the noise reducer performs noise reduction processing with less image lag.
As described above, X-ray doses used to take X-ray fluoroscopic images are extremely low. Thus, a huge number of noise signals due to quantum noises are superposed on each taken X-ray fluoroscopic image. Accordingly, noise reduction processing using recursive filtering is indispensable.
In a case where a conventional technique is applied to an X-ray fluoroscopic imaging apparatus having a frequency processing function, recursive filtering can be applied to a high-frequency component image obtained by a frequency component decomposition function. However, a frame memory for delaying image data by 1 frame is necessary for performing recursive filtering. In a frequency processing analysis, sometimes, an original image is reconstructed from decomposed image data after predetermined processing is performed on decomposed image data respectively corresponding to a plurality of frequency bands, into which original image data is decomposed. Accordingly, delay memories for delaying decomposed image data are needed.
That is, in the case where the conventional technique is applied to the X-ray fluoroscopic imaging apparatus having a frequency processing function, the X-ray fluoroscopic imaging apparatus requires a frame memory for delaying the original image data by 1 frame, and delay memories for delaying the decomposed image data. Consequently, in the case where the conventional technique is applied to the X-ray fluoroscopic imaging apparatus having the frequency processing function, the required memory capacity and an amount of memory access in the entire X-ray fluoroscopic imaging apparatus are dramatically increased.
In a case where images, on which frequency processing is performed, are stored in an integrated large capacity memory, an amount of access to this memory is extremely increased. In a case where recursive filtering is additionally performed using the integrated large capacity memory, an amount of memory access is considerably increased.